The present invention relates generally to radio frequency identification systems, particularly to the reader of a radio frequency identification system, and more particularly to a reader having a feedback circuit which enables the reader to automatically adjust to variations in the operating environment of the reader.
Radio frequency identification (RFID) systems generally consist of a reader, also known as an interrogator, and a remote tag, also known as a transponder. The reader is termed an active device, while the tag is termed a passive device insofar as the tag lacks an internal power source, relying on the reader to remotely power its operation by inductive coupling. The RFID system is designed to provide communication between the reader and the remote tag in a wireless and contactless manner using radio frequency (RF) signals within a relatively narrow frequency range at a low or medium frequency, i.e., below about 30 MHz. Since the reader is active and the tag is passive, the reader must initiate all communications between the tag and the reader with the tag responding to the initiative of the reader.
Communication between the reader and tag is enabled by corresponding resonant LC pairs provided in the reader and tag which magnetically couple through the mutual inductance of the respective reader and tag inductors. The reader and tag inductors are coils which each function as transmitter and receiver antennas. An exemplary RFID system of the type described above is disclosed in U.S. Pat. No. 4,730,188 to Milheiser. Alternatively, the RFID system may have separate transmitter and receiver antennas in either the reader or the tag, or in both.
Communication between the reader and tag is initiated by the reader when the tag is proximally positioned relative to the reader. A current is conveyed to the reader LC pair causing the reader inductor to produce an excitation signal in the form of an electromagnetic field. The excitation signal couples to the proximally-positioned tag coil through mutual inductance and the excitation signal powers and clocks the tag circuitry initiating operation of the tag. Tag operation comprises generation of a response signal and transmission of the response signal from the tag back to the reader. In particular, a current is conveyed from the tag circuitry to the tag LC pair in response to the excitation signal causing the tag inductor to produce a response signal in the form of an electromagnetic field. The response signal couples to the reader inductor through mutual inductance in substantially the same manner as described above with respect to coupling of the excitation signal to the tag inductor. The tag typically employs frequency or amplitude modulation of the response signal to encode data stored in the memory of the tag circuitry into the response signal. When the response signal couples to the reader inductor, a corresponding current is induced in the reader inductor. The reader processes the induced current to read the data encoded in the response signal. The resulting data may be communicated to an output device, such as a display, printer, communication, or storage device, and simultaneously, or alternatively, communicated to a host computer, if a host computer is networked into the RFID system.
A common application for RFID systems of the type described above is security access control, wherein an authorized individual is provided with a tag typically mounted in a card. To gain access to a secured area, the authorized individual places the card near the reader enabling the reader to read identifying information stored in the memory of the tag and determine that the individual is authorized to enter the secured area. Another application for RFID systems is tracking living things or goods. A tag is applied to the object being tracked and a reader is placed at one or more locations where the object is known to pass. Movement of the object is recorded whenever the object passes a reader.
An important operational parameter of the reader is the range of the reader for communication with the tag, particularly when the reader is utilized in tracking applications. The range of the reader is inter alia strongly affected by the strength of the electromagnetic field generated by the reader LC pair. In order to generate a field strength which provides the reader with adequate range, the designer of the reader must properly specify a resonant circuit which is appropriately tuned for the desired application of the RFID system. An important design parameter for specifying the tuned resonant circuit is the quality factor, Q, which is simultaneously a measure of bandwidth and circuit efficiency. Circuit Q is usually limited by the maximum current the designer wishes to allow through the resonant circuit and the practical limitations of component values generally available for volume production. Another important design parameter for specifying the tuned resonant circuit is the resonant frequency, which is determined by the exact values of the components installed in the resonant circuit. In the event that modulation of the excitation signal is contemplated, additional constraints may be specified for the resonant circuit. A number of means exist to address these constraints, some of which, but not all, involve further limiting circuit Q.
In view of the above, the resonant circuit is generally designed by specifying the nominal resonant frequency of the circuit. Nominal inductive and capacitive components are determined which provide optimal performance for the desired application of the RFID system. The designer then determines the allowable circuit Q by calculating or measuring the effect of several factors, including the cost-effectiveness of specifying certain tolerances on components, drift of component values with time and temperature, nonlinearity of components (especially certain inductors) with operating voltages or currents, and the effect of metals or other electrically active materials in the immediate operating environment of the reader. For example, nearby metal in the operating environment of the reader may cause inductors of the type used in resonant circuits to change their effective inductance, thereby detuning the resonant circuit and reducing the range of the reader dramatically.
Accordingly, it is important that the design of the reader is robust in the sense that the anticipated influence of the above-recited factors does not unduly degrade the range of the reader. The usual design methodology to achieve robustness is to compromise circuit Q so that the resonant area of the resonant circuit is sufficiently broad. Unfortunately, this compromise results in decreased circuit efficiency, resulting in undesirable requirements such as higher power supply voltages and increased power consumption. This is especially undesirable in RFID systems powered by batteries, solar cells or other limited power sources. It also typically adds to the cost of the RFID system, even if power availability is not a significant limitation.
The present invention recognizes a need for a reader of an RFID system which is adaptable to a broad range of applications and their corresponding operating environments. Accordingly, it is an object of the present invention to provide an RFID system with a reader which performs effectively in a variety of applications. More particularly, it is an object of the present invention to provide an RFID system with a reader exhibiting satisfactory performance characteristics which adjust to variations in a given operating environment of the reader. It is another object of the present invention to provide a reader achieving a uniformly satisfactory level of performance when the reader is employed in different operating environments. It is another object of the present invention to provide a reader which automatically retunes itself to maintain a desired performance level in response to variations in a given operating environment or in response to relocating the reader to a different operating environment. These objects and others are accomplished in accordance with the invention described hereafter.
The present invention is a reader for an RFID system. The reader comprises an exciter circuit for generating an excitation signal and a feedback circuit coupled to the exciter circuit for automatically tuning the exciter circuit. The exciter circuit has at least one retunable component providing the exciter circuit with adjustable component values and a plurality of signal generating states. The retunable component may, for example, be a varactor, switched discrete capacitor, or an electrically adjustable inductor. The exciter circuit is initially tuned to a first signal generating state, but is retunable to additional signal generating states by adjusting the component value of the retunable component.
The feedback circuit includes a circuit evaluator coupled to the exciter circuit for determining a value of an operational parameter of the exciter circuit. The circuit evaluator may, for example, be a current detector or a voltage detector. A decision-making circuit is coupled to the circuit evaluator for formulating a decision in response to the value of the operational parameter. The decision-making circuit may, for example, be an analog device or a digital state machine. An adjustment circuit is coupled to the decision-making circuit and exciter circuit for receiving the decision and conveying an adjustment instruction to the exciter circuit in response to the decision.
The present invention is additionally a method for automatically tuning the exciter circuit of the reader to a specific operating environment by employing the feedback circuit described above. The method includes generating an excitation signal using the exciter circuit tuned to a first signal generating state. An operational parameter of the exciter circuit is evaluated at the first signal generating state by measuring a first value of the operational parameter. The operational parameter may, for example, be current or voltage in the exciter circuit. A second signal generating state for the exciter circuit is decided in response to the evaluation step by determining a second more optimal value of the operational parameter and comparing the first value to the second value. The exciter circuit is retuned to the second signal generating state in response to the decision step. The generation, evaluation and decision steps may be repeated at the second signal generating state and the exciter circuit is retuned to a third signal generating state in response to the decision.